Host Discovery

HostDiscoveryResponder.java

import java.io.IOException;
import java.net.*;

public class HostDiscoveryResponder implements Runnable {
    DatagramSocket socket;

    public static void main(String[] args) {
        System.setProperty("java.net.preferIPv4Stack", "true");
        Thread discoveryThread = new Thread(new HostDiscoveryResponder());
        discoveryThread.start();
    }

    @Override
    public void run() {
        try {
            socket = new DatagramSocket(8888, InetAddress.getByName("0.0.0.0"));
            socket.setBroadcast(true);

            while (true) {
                //byte[] recvBuf = new byte[256];
                byte[] recvBuf = "DISCOVERY_REQUEST".getBytes();
                DatagramPacket packet = new DatagramPacket(recvBuf, recvBuf.length);
                socket.receive(packet);

                //String message = new String(packet.getData()).trim();
                String message = new String(packet.getData());
                if (message == "DISCOVERY_REQUEST") {
                    byte[] sendData = "DISCOVERY_RESPONSE".getBytes();

                    DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, packet.getAddress(), packet.getPort());
                    socket.send(sendPacket);
                }
            }
        } catch (SocketException e) {
            e.printStackTrace();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

HostDiscoverySender.java

import java.io.IOException;
import java.net.*;
import java.util.ArrayList;

public class HostDiscoverySender implements Runnable {
    private DatagramSocket socket;
    public ArrayList<InetAddress> hosts;
    public boolean waitForResponse;

    public static void main(String[] args) {
        System.setProperty("java.net.preferIPv4Stack", "true");
        Thread discoveryThread = new Thread(new HostDiscoverySender());
        discoveryThread.start();
    }

    @Override
    public void run() {
        hosts = new ArrayList<InetAddress>();
        waitForResponse = true;

        try {
            socket = new DatagramSocket();
            socket.setBroadcast(true);

            while (waitForResponse) {
                byte[] sendData = "DISCOVERY_REQUEST".getBytes();
                DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, InetAddress.getByName("255.255.255.255"), 8888);
                socket.send(sendPacket);

                byte[] recvBuf = "DISCOVERY_RESPONSE".getBytes();
                DatagramPacket packet = new DatagramPacket(recvBuf, recvBuf.length);
                socket.receive(packet);

                hosts.add(packet.getAddress());
                System.out.println(packet.getAddress());
            }
        } catch (SocketException e) {
            e.printStackTrace();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}